1. Field of the Invention
The present invention relates to a temperature compensated piezoelectric oscillator and an electronic device including the temperature compensated piezoelectric oscillator. More particularly, the present invention relates to a temperature compensated piezoelectric oscillator including a variable capacitance diode for varying the oscillation frequency, and to an electronic device including the temperature compensated piezoelectric oscillator.
2. Description of the Related Art
FIG. 6 shows a circuit diagram of a temperature compensated crystal oscillator, which is one type of conventional temperature compensated piezoelectric oscillator. In FIG. 6, only the main portions are shown. An example of the entirety of a known temperature compensated piezoelectric oscillator is disclosed in Japanese Unexamined Patent Application Publication No. 2002-135053.
In FIG. 6, a temperature compensated crystal oscillator 1 includes a thermistor network 2 formed of a circuit network composed of a transistor Q1 which is an active element for oscillation, a crystal vibrator X1 which is a piezoelectric vibrator that functions as a resonance circuit, a variable capacitance diode VD1, a resistor, and a thermistor, resistors R1 to R6, and capacitors C1 to C5. Furthermore, the temperature compensated crystal oscillator 1 includes power-supply terminals Vcc1 and Vcc2, an external frequency control terminal Vc, and an output terminal Po.
Here, the collector of the transistor Q1 is connected to the power-supply terminal Vcc1 and is connected to ground via the capacitor C4. The emitter thereof is connected to ground via both the resistor R1 and the capacitor C2. The base thereof is connected to one end of the crystal vibrator X1. The capacitor C4 is set so that the impedance becomes sufficiently low at the oscillation frequency in such a manner that the collector of the transistor Q1 is grounded at high frequencies at the oscillation frequency so as to cause the transistor Q1 to perform a collector grounding operation. The capacitor C1 is connected between the base and the emitter of the transistor Q1, and the capacitor C1, together with the transistor Q1, the capacitor C2, and the crystal vibrator X1, forms a Colpitts oscillation circuit. The base of the transistor Q1 is connected to the power-supply terminal Vcc1 via the resistor R2 and is connected to ground via the resistor R3. The emitter of the transistor Q1 is connected to the output terminal Po via the capacitor C3.
On the other hand, the cathode of the variable capacitance diode VD1 is connected to the other end of the crystal vibrator X1, and the anode thereof is connected to ground via the capacitor C5. The capacitor C5 is set in such a manner that the impedance becomes sufficiently low at the oscillation frequency in order to cause the anode of the variable capacitance diode VD1 to be high-frequency grounded at the oscillation frequency. For an ideal capacitor, the larger the electrostatic capacitance, the smaller the impedance becomes. However, since an actual capacitor contains parasitic inductance components, etc., it does not necessarily function as a capacitor at the frequency higher than or equal to the self-resonant frequency. Therefore, for the capacitor C5, a multilayer chip capacitor of 1000 pF, in which the self-resonant frequency is higher than approximately 20 MHz, which is the oscillation frequency of the temperature compensated crystal oscillator 1, is used. In this case, the impedance at the oscillation frequency becomes approximately 8Ω, and it is possible to provide the function of causing the anode of the variable capacitance diode VD1 to be grounded at the oscillation frequency.
The anode of the variable capacitance diode VD1 is connected to an external frequency control terminal Vc via the resistor R4, which is the first resistor, and is also connected to the ground via the resistor R5 which is the second resistor. In other words, the voltage of the external frequency control terminal Vc divided by the resistors R4 and R5 is applied to the anode of the variable capacitance diode VD1.
Then, the cathode of the variable capacitance diode VD1 is connected to the thermistor network 2 via the resistor R6. The thermistor network 2 is a three-terminal circuit network formed of resistors and thermistors, with one terminal (electrical-current input terminal) being connected to the power-supply terminal Vcc2, which has a voltage stability that is higher than that of the power-supply terminal Vcc1, another terminal (electrical-current output terminal) being connected to ground, and another terminal (voltage output terminal) being connected to the resistor R6. Then, a voltage such that the voltage applied from the power-supply terminal Vcc2 between the electrical-current input terminal and the electrical-current output terminal is divided by a resistor and a thermistor is output from the voltage output terminal. A bypass capacitor for eliminating noise in the power supply may be connected between the power-supply terminal Vcc2 and the ground. Since the crystal vibrator X1 and the cathode of the variable capacitance diode VD1 are connected before the resistor R6, DC electrical current does not flow through the resistor R6. Since the voltage stability of the power-supply terminal Vcc1 is sufficiently high, the electrical-current input terminal of the thermistor network 2 may be connected to the power-supply terminal Vcc1.
The temperature compensated crystal oscillator 1 configured as described above oscillates at a frequency which is almost determined by the electrostatic capacitances of the crystal vibrator X1, the capacitors C1 and C2, and the variable capacitance diode VD1, and an oscillation signal is output from the output terminal Po. The oscillation frequency varies according to the electrostatic capacitance of the variable capacitance diode VD1, in other words, the bias voltage applied between the cathode and the anode of the variable capacitance diode VD1. In the thermistor network 2, since the voltage to be output from the voltage output terminal varies with temperature, the cathode voltage of the variable capacitance diode VD1 varies, and the electrostatic capacitance of the variable capacitance diode VD1 varies, with the result that the oscillation frequency of the temperature compensated crystal oscillator 1 also varies. By forming the thermistor network 2 so that the variation of the output voltage of the thermistor network 2 nearly cancels the variation of the resonance frequency of the crystal vibrator X1 due to the temperature variation, the temperature compensated crystal oscillator 1 is able to maintain the oscillation frequency regardless of the temperature.
The anode voltage of the variable capacitance diode VD1 is such that the control voltage applied to the external frequency control terminal Vc is divided by the resistors R5 and R4. For this reason, also, by varying the control voltage, the electrostatic capacitance of the variable capacitance diode VD1 varies, and the frequency oscillation of the temperature compensated crystal oscillator 1 varies.
In the temperature compensated crystal oscillator used as a reference signal source in a mobile phone, the external frequency control terminal is used to correct the oscillation frequency during communication with the base station.
In the temperature compensated crystal oscillator 1, since the variable capacitance diode VD1 is used to vary the frequency, the following problems exist.
First, the cathode of the variable capacitance diode VD1 is connected to the thermistor network 2. The thermistor network 2 is a three-terminal circuit network formed of resistors and thermistors, in which the voltage applied between the electrical-current input terminal and the electrical-current output terminal is divided and is output so as to be applied to the cathode of the variable capacitance diode VD1. At this time, DC electrical current flows through the resistors and thermistors. When electrical current flows through the resistors (also including the thermistors), electrical-current noise having substantially 1/f frequency characteristics is generated, and this is applied to the cathode of the variable capacitance diode VD1. In some resistors, for example, thick-film resistors, the magnitude of the electrical-current noise generated as a result of electrical current flowing through the resistors is considerably greater than that of the thermal noise generated from the resistors. When such electrical-current noise is applied to the variable capacitance diode VD1, the oscillation signal is FM-modulated by this noise, and phase noise, particularly at low detuning frequencies of the oscillation signal, is reduced.
On the other hand, the anode of the variable capacitance diode VD1 is also connected to the connection point of the resistors R5 and R4. The resistors R5 and R4 are used to divide the voltage applied to the external frequency control terminal Vc and to apply the divided voltage to the anode of the variable capacitance diode VD1. Naturally, DC electrical current flows therethrough, and electrical-current noise occurs similarly to the case of the thermistor network 2, causing the phase noise of the oscillation signal to be reduced. Since the capacitor C5 connected between the anode and the ground of the variable capacitance diode VD1 is set so as to have a sufficiently low impedance at the oscillation frequency and the self-resonant frequency thereof is higher than a low frequency which affects the phase noise, it is not useful for bypassing and reducing the electrical-current noise at low frequencies.
Since electrical-current noise is proportional to the resistance value of the resistance network and the electrical current flowing therethrough, it is made proportional to the voltage applied to the resistance network. Electrical-current noise corresponding to the voltage of the cathode of the power-supply terminal Vcc2 is applied to the cathode of the variable capacitance diode VD1, and electrical-current noise corresponding to the voltage applied to the external frequency control terminal Vc is applied to the anode. If it is assumed that the voltage of the power-supply terminal Vcc2 and the voltage applied to the external frequency control terminal Vc are equal to each other, it is considered that the power of the electrical-current noise is greater by an amount equal to the square root of two (approximately 1.4 times greater) than the case where either one of them exists. Naturally, this causes the phase noise of the oscillation signal to be increased.
In the manner described above, the conventional temperature compensated piezoelectric oscillator experiences the problem of an increase of the phase noise, resulting from the electrical-current noise applied to the variable capacitance diode.